[Commonly used optical components] Quality parameters of optical components (1)

[Commonly used optical components] Quality parameters of optical components (1)

Surface Quality:

The surface quality of optical components mainly describes its appearance, especially defects such as scratches and pits. Such defects can lead to local energy accumulation and an increase in scattered light, and some optical components are very sensitive to these effects. For example:

(1) On the surface of optical elements used for imaging, such defects will greatly affect the imaging quality;

(2) On optical elements used in high-power laser optical paths, such defects will cause local accumulation of energy and cause damage to the optical elements.

The most commonly used evaluation standard for surface quality is Scratch/Dig. Scratch can be simply understood as scratch-like defects on the surface of the lens, which is specifically characterized as Scratch number, which is the maximum width of the flaw; dig can be simply understood as the pit defects on the surface of the lens, which is specifically characterized as Dig number, which is defined as the average of the maximum pit length and the maximum pit width. For example, a surface quality of 20/10 means that the maximum size of scratches, that is, scratch-like defects, does not exceed 20 μm, and that the maximum size of dig, that is, pit-like defects, does not exceed 100 μm.

Figure 1. How Scratch and dig are defined

The following are several common surface quality grades: 120/80: industrial polishing grade; 60/40: commonly used scientific research grade; 40/20: laser application grade; 20/10: precision optical grade

Surface flatness:

Surface flatness is a standard that measures the deviation of the surface flatness of optical components. It is a very important evaluation parameter for mirrors, windows, prisms or lenses, etc. Optical elements with poor surface flatness will cause wavefront distortion of the incident light, leading to aberrations, which will have a very adverse impact on both imaging applications and optical transmission. If a lens with substandard flatness is mixed into the experimental optical path, the resulting distortion will be amplified by subsequent optical paths, making it more difficult to eliminate. Surface flatness is generally measured using a high-quality, high-precision optical plane as a reference plane, using the principle of interference (the most commonly used commercial interferometer product at present is the American Zygo interferometer). When the flat surface of the test optic is placed on an optical plane, interference fringes are seen to appear due to the interference effect, the shape of which determines the surface flatness of the optic being inspected. If the stripes are equally spaced parallel straight strips, then the optical surface being tested is at least as flat as the reference optical plane; if the stripes are curved, use two dotted lines as a reference. One of these two dotted lines is tangent to the center of the stripe, and one passes through the end of the same stripe. The number of stripes between the dotted lines represents the flatness error.

Figure 2. Definition of surface flatness

Figure 3. Measurement interface of Zygo interferometer

Deviation from flatness is usually measured in units of wavelength (λ), which is a multiple of the wavelength of the test source. It is generally defined that a stripe represents 1/2λ, and we often see parameters such as λ/4 or even λ/20. This parameter is obtained by considering the curvature of the stripe. A flatness of 1λ is considered the most basic grade, while a flatness of λ/4 is considered an ordinary precision grade, and λ/20 is considered a high precision grade. One thing to note here is that for coated optical components, the surface flatness parameters indicated on the products provided by most manufacturers are actually the parameters before coating, that is, the surface flatness of the glass substrate. Only a few manufacturers can guarantee the surface flatness after coating (this is because the surface flatness after coating will be affected, and manufacturers who are more confident in coating technology can minimize this impact). When purchasing, remember to distinguish or ask the manufacturer whether the flatness parameter given is before plating or after plating.

Deviation from flatness is usually measured in units of wavelength (λ), which is a multiple of the wavelength of the test source. It is generally defined that a stripe represents 1/2λ, and we often see parameters such as λ/4 or even λ/20. This parameter is obtained by considering the curvature of the stripe. A flatness of 1λ is considered the most basic grade, while a flatness of λ/4 is considered an ordinary precision grade, and λ/20 is considered a high precision grade. One thing to note here is that for coated optical components, the surface flatness parameters indicated on the products provided by most manufacturers are actually the parameters before coating, that is, the surface flatness of the glass substrate. Only a few manufacturers can guarantee the surface flatness after coating (this is because the surface flatness after coating will be affected, and manufacturers who are more confident in coating technology can minimize this impact). When purchasing, remember to distinguish or ask the manufacturer whether the flatness parameter given is before plating or after plating.